1. Field of the Invention
The present invention relates to electrolytic capacitors and, more particularly, to the use of separator material to encapsulate anode assemblies or to encapsulate stacked capacitor configurations for use with electrolytic capacitors. The present invention also relates to electrolytic capacitors comprising the anode assemblies or stacked capacitor configurations of the present invention.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors typically consist of a cathode electrode, an electrically conductive electrolyte and a porous anode with a dielectric oxide film formed thereon. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
The need for high voltage, high energy density capacitors is most pronounced when employed in implantable cardiac defibrillators (ICDs). In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number and volume of the anodes. A multiple anode flat, stacked capacitor configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. In order to achieve higher energy densities, three, four and five anodes can be stacked per layer. Maximization of the anode volume has also been accomplished by etching to achieve more effective anode surface area, and making the relative size of the anode plates larger with respect to the cathode plates.
Increasing the size of the anodes creates another problem, however, as it is still necessary to electrically insulate the anode plates from the case, which typically is grounded to the cathode assembly. Insulation between the anodes and the case can be accomplished using an insulating tape such as KAPTON® or TEFLON® (both available from DuPont). Taping is generally best performed with an adhesive applied, so as to hold the stack together during the assembly process, securing the tape to prevent interference with welding, and providing positive evidence of insulation between case and anode edge, without gaps to compromise the desirable insulating properties.
This solution has proven to be problematic though, as the adhesive tape can interfere with the aging process of the anode edges. Aging is performed to assure that there is sufficient dielectric present on raw metal portions of the anode to allow for adequate charge storage with low leakage and short charge times to voltage. However, adhesive covering the edge of the anodes can prevent fill electrolyte (the aging medium) from reaching the raw aluminum surfaces, resulting in areas of the anodes that have insufficient dielectric voltage during fast charging and discharging. If the adhesive partially or completely dissolves into the electrolyte, these surfaces become exposed to fresh electrolyte. In ICDs used for pulse discharge applications, during the charging process, this can result in abnormally long charge times, due to the fact that charging energy is consumed to age these newly exposed areas in order to sufficiently store charge. This effect is particularly pronounced when the battery open circuit voltage drops due to a change in battery chemistry at mid-life.
Some methods to alleviate this problem include: application of a layer of spacer in between the tape and anode edge; using tape with no, minimal, or patterned adhesive; and withdrawing the anodes to inside the cathode layers and thereby preventing adhesive from contacting the edges. These solutions all present drawbacks either to manufacturability, or result in a decrease in energy density due to decreased anode volume. As a result, there is a need to provide an electrolytic capacitor with a large anode volume to produce a high energy density, while still maintaining greater total manufacturability.